The present disclosure relates generally to the problem of achieving fast drying of thermal inkjet ink on various substrates.
The adhesion of an ink film on a substrate depends, at least in part, on the interaction of the ink components with the substrate. In semi-porous and non-porous substrates the adhesion of the ink is significantly influenced by the properties of the media at the microscopic level. These properties include the surface energy of the substrate, roughness of the substrate, and porosity of the substrate. Poor adhesion of the ink to the substrate can be manifested when an ink can be easily removed or abrased by an object. On many occasions, the abradant or the abrasive object is represented by the media itself, but in other cases more abrasive objects can also be present.
Poor resistance to abrasion may be related to the adhesion of the ink film to the substrate. In general terms, if the ink is not adequately adhered to the substrate, then the ink is less resistant to abrasion. Thus, to improve the adhesion of the ink to the substrate, and consequently obtain improved resistance to abrasion, one might need to maximize the anchoring of the ink to the substrate. A way to improve anchoring of the ink to the substrate might include the use of corona discharge, among other treatments. This treatment applies a corona discharge, which results in the production of ozone, which is the same highly reactive species which causes chemical transformations (e.g., oxidation) of the substrate surface. With this treatment, the surface comes to contain chemical groups that can interact with the ink components (via Van der Waals, hydrogen-bonding interactions, etc.) and form a strong “bond” with the ink. See Tran et al., U.S. Patent Application Publication No. 200410055698A1.
An alternative to improve the adhesion of an ink film to the substrate is via the use of a binder resin. The binder resin can interact with the substrate and form a stronger bond with the substrate, which can help improve the ink adhesion, and consequently, the resistance to abrasion. In addition, the mechanical properties of the polymeric binder resin influences the abrasion resistance of the ink film, as well. For example, glass transition temperature (Tg) and molecular weight of the polymer affect its mechanical properties.
Prior solutions for improving ink durability, especially abrasion resistance, have been discussed above. One solution discussed was the use of corona treatment. This treatment, though very effective under certain circumstances, is generally not appropriate for high-speed applications, such as in industrial applications that require a high throughput, such as in the presently disclosed application.
The use of resins in ink formulations was also discussed above. Resins have been reported as an additive to improve the durability of printed inks on media. The ink formulations reported to contain resins are usually highly viscous solutions (higher than 10 centipoise seconds (cPs)). Such formulations are not compatible with thermal ink jet technology due to the fluidic limitations imposed by that technology. Solvent-based inks containing resins are, however, well known in the art of Continuous Inkjet (CIJ) and Piezoelectric Inkjet (PIJ) technologies. Such inks used in CIJ and PIJ usually have a viscosity of approximately 10-14 cPs and are therefore not compatible with thermal inkjet. Under most circumstances, thermal inkjet can effectively operate with fluids having a viscosity in the range of 1 to 4 cPs. Hence the resin-containing solvent-based inks used in CIJ or PIJ are inappropriate for thermal inkjet.
Binder resins have also been reported in thermal inkjet systems. Such resins have been used in underprinting or overprinting fluids which are printed separately in time and underneath or over the top of water-based ink images. The binder resin in the underprinting or overprinting fluid interacts with the colorant in the ink that is applied separately. TIJ inks have also been reported that contain binder resins in the ink itself. The overall effect is to bind the ink colorant not only to the polymer resin but to the substrate. Most binder resins are however substantially incompatible with a water-based ink and thus are not an actual component of the ink but are contained in a separate fluid.